The present invention relates to an elliptic beam antenna used in various kinds of radar, satellite communications, satellite broadcasting and terrestrial radio communications.
In the fields of radar, satellite communication and satellite broadcasting, antennas of the type radiating elliptic beams are often used with a view to achieving such effects as mentioned below. First, in a case of radar, a beam width in the direction of scanning is made smaller than in a direction crossing it at right angles so as to provide enhanced angular resolution in the direction of scanning. In the direction perpendicularly intersecting that of scanning, a cosecant beam having a slightly raised level at the bottom of the radiation pattern may sometimes be used with a view to compensate for a far-near distance effect.
Second, satellite antennas for satellite communications or satellite broadcasting sometimes employ an elliptic beam rather than a circular one for the purpose of high efficiency coverage of service areas. In a case of earth station antennas, to reduce the amount of interference between adjacent satellites, the minor axis of the elliptic beam is pointed toward the direction of orbit of a geostationary satellite to make the level of the sidelobe in this direction lower than in case of using the circular beam. This scheme, in some cases, omits antenna tracking of the direction off diurnal variations in the position of the satellite by perturbation.
Incidentally, an efficient elliptic beam cannot be generated by an ordinary rectangular, circular or elliptical aperture horn alone. FIG. 7 shows this; as in a case of a square or circular aperture horn, the directions of the major and minor axes of the elliptic beam shift with each other according to the kind of polarized wave for excitation. In case of the elliptic aperture horn, also, the flatness changes with the kind of polarized wave for excitation; that is, the one polarization produces an elliptic beam but the other polarization a circular beam. The irradiation of an elliptical service area or reflecting mirror with such beams will cause an spillover of radio waves, and hence is inefficient in terms of antenna gain, besides degradation of the side lobe characteristic by the spillovering radio waves leads to an increase of interference between the system concerned and other systems.
Since the circularly polarized wave which is used in radar is obtained by synthesizing the above-mentioned horizontally polarized and vertically polarized waves, the excitation by the circularly polarized wave cannot generate an efficient elliptic beam either; furthermore, there are cases where a ghost is formed or resolution is impaired by the side lobe.
That is, the elliptic beam can efficiently be generated only by the combined use of a reflecting mirror with an elliptical aperture and a feeder system which emits the elliptic beam.
Hence, there have been employed various feeder systems, such as listed below, in combination with an elliptical reflector.
(1) Waveguide array or the like (FIG. 8) PA1 (2) Corrugated conical horn and modified sub-reflector (FIG. 9) PA1 (3) Elliptical corrugated horn (FIG. 10)
The combination (1) is used to generate a difference pattern as well as a sum pattern mainly in the field of radar. The combination (2) is used in small earth station antennas and (3) in satellite antennas.
The prior art schemes (1) to (3) cannot attain their objects without making full use of complex and sophisticated design/fabrication technologies. That is, the array such as a waveguide (1) must be designed/manufactured taking into account such conditions as horn size/horn number tradeoffs and the branching (multiplying) accuracy of a feeding circuit. The corrugated horns (2) and (3) are not suited to mass production as compared with ordinary circuit aperture horns and rectangular aperture horns. The elliptical corrugated horn is more difficult to design and fabricate than the conical corrugate horn. Furthermore, it is also necessary to produce a main reflector and a sub-reflector that are needed in (2), by modification from a quadratic surface such as a paraboloid or hyperboloid.
For the reasons given above, conventional antennas are expensive.